This Recommendation contains functionality to describe the use of the real-time transport protocol control protocol (RTCP) in H.248-controlled media gateways. RTCP is used for instance to monitor the quality of service and to convey information about the participants in an on-going real-time transport protocol (RTP) session.

[ITU-T H.248.1] assumes that when an RTP stream is specified in a local or remote descriptor, that an RTCP flow may be established according to the rules defined in clause 11 of [IETF RFC 3550]. Additional H.248/RTCP behaviour is defined in clause E.12.5 of [ITU-T H.248.1]. However, some media gateways may not instantiate an RTCP flow. For interoperability and for some applications such as firewall and network address translation (NAT), it is important for the MGC to be certain of the media gateway behaviour with respect to port allocation for RTCP. The "RTCP Handling Package" defined in this Recommendation provides a property to control this RTCP flow allocation.

Overview

RTCP handling may comprise the following functions:

1) Indication of the existence of an RTCP flow of an RTP session (RTP with or without RTCP).

2) Indication of endpoint identifiers on RTP level that are relevant for both RTP and RTCP (e.g., SSRC).

In addition, if an RTCP flow exists:

3) Resource management of endpoint identifiers on layer 4 (UDP ports; i.e., reservation, allocation and indication of a specific allocation rule) for RTCP flows.

4) Resource management of endpoint identifiers on RTP level (e.g., CNAME) for RTCP flows.

5) Resource management of other resources required for RTCP flows (e.g., reservation and allocation of transport capacity).

6) Indication of specific modes of operation for RTCP (e.g., layer 4 multiplexing of RTP and RTCP flows, or multiplexing mode of RTCP reports in a single RTCP packet).

Orthogonal to the above functions:

7) Mapping scheme of RTP and RTCP flows on H.248 streams.

This Recommendation deals with functions 1 and 3 only. Other functions are out of the scope of this Recommendation.

In addition, this Recommendation assumes that regarding function 7, RTP and RTCP flows are always mapped into a single H.248 stream. Other mapping schemes are out of the scope of this Recommendation and may make this Recommendation's procedures irrelevant.